Many car engine components are driven from the rotating forces of the engine via a belt. In older cars, several belts were used. Most modern cars use a single, serpentine belt to drive all the necessary parts of the engine. Usually this consists of the power steering motor, alternator, water-cooling pump and air-conditioning unit. The belt requires consistent tension in order for it to efficiently drive all the components. Most cars use a spring-loaded belt tensioner to keep the serpentine belt pulled tight. If this goes bad, it can lead to many car problems.
Listen for squealing noises within the engine when it is running. While there are many causes for squealing sounds within an engine, a bad belt tensioner is a likely culprit. Usually you will hear the squealing when the engine is idling at a low speed, and it will fade away as the engine revs up.
Watch the belt and belt tensioner while the engine is running. A bad belt tensioner spring will often cause the tension arm to bounce up and down with the cycling of the motor. This makes the belt wobble as the engine runs. These are definite signs that the tensioner needs replaced.
Turn off the engine. Examine the serpentine belt for unusual damage. If your engine's belt has broken or cracked grooves, is worn smooth or is fraying then the belt will need to be replaced. A bad belt tensioner is a likely cause of belt damage. However, if your belt is old and has not been replaced in several years, it might just be normal wear and tear.
Push down on the middle of the belt while the engine is off and the components are cool enough to touch. If you are able to push the belt down more than about an inch, the tensioner probably needs to be replaced. A good tensioner should be holding the belt snugly in place with enough resistance not to allow you to push it down that far.
Check your owner's manual for a calendar of how often your serpentine belt should be replaced.
Don't reach into the engine compartment while the car is on. If you are unsure whether or not the belt tensioner needs replaced, take the car to a mechanic.